It is likely that someone at the Internal Revenue Service illegally leaked confidential donor information showing a contribution from Mitt Romney’s political action committee to the National Organization for Marriage, says the group.

In a letter exclusively obtained by The Daily Caller, the organization has asked the Treasury Inspector General for Tax Administration to “investigate and learn the source of the breach and prosecute those responsible to the fullest extent of the law.” It also provided evidence suggesting the leak might have been the result of “IRS employee misconduct.”

The dispute stems from a March 30 post on the pro-gay Human Rights Campaign’s (HRC) website, which boasted that it had obtained documents revealing that Mitt Romney “donated $10,000 to the National Organization for Marriage [NOM] in 2008 …”

NOM argues that such an unauthorized public release of confidential taxpayer information was a clear violation of federal law.

Tax-exempt organizations like NOM are required file Form 990 with the IRS annually, which includes a schedule of donors who contribute at least $5,000 during a reporting period. This information is filed solely with the IRS, and the Internal Revenue Code requires the agency to keep the information confidential.

NOM has been crying foul since the information was revealed, but it is now taking formal legal steps. Excerpts from the letter (signed by NOM’s chairman Dr. John C. Eastman and president Brian Brown) explain why NOM believes that someone at the IRS might be the culprit:

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NOM has also issued a demand letter to both the HRC and the Huffington Post (which also published the documents), informing them that publicly revealing these confidential donors constitutes a “felony.” The letter also instructs both organizations to remove the names and links to their private donors from their website and their archives within 24 hours.

Lastly, NOM is insisting the outlets name the leak.

The story could have larger implications: Not only does it involve Romney, but outgoing Human Rights Campaign president Joe Solmonese was recently named a national co-chair of the Obama presidential re-election campaign.

During a conversation with TheDC, a spokesman for NOM stressed the issue isn’t about gay marriage, but rather the rule of law. The notion that confidential donor information can be compromised with impunity, the spokesman said, could have a chilling effect on contributions to such groups. Some donors are more comfortable making anonymous donations because they fear possible reprisal or intimidation; in other cases, they merely seek privacy.

The published record indicates that NOM is fighting for principle, not to protect the anonymity of any specific donation. The Deseret News reported on the $10,000 donation in 2008, and confirmed it through Romney spokesman Eric Fehrnstrom. And the Huffington Post’s reporting includes a link to a public campaign-finance document showing the donation, made — for reasons that aren’t clear — through the PAC’s Alabama chapter.

With the Romney PAC’s donation already public, NOM’s message boils down to a complaint about a potential IRS leak. And the leaker, the organization says, is not a “whistleblower” (as some have suggested), but a criminal.